Seasonal partitioning of nitrogen by plants and soil microorganisms in an alpine ecosystem

The seasonal dynamics of plant N assimilation and microbial N immobilization were studied in an alpine ecosystem to evaluate temporal patterns of plant and microbial N partitioning and the potential for plant vs. microbial competition for N. Plant N uptake was higher in the first half of the growing season than later in the season, as indicated by changes in biomass N and by ¹⁵N uptake. Microbial N pools were low during the first half of the growing season (9.5 g N/m² on 1 June) and increased late in the season, from 11.4 g N/m² on 1 August 1991 to 38.6 g N/m² on 14 October 1991. Two different measures of N availability were highest in the midseason. Ion exchange resin bag N uptake was greatest in July (86.0 μg N · g^-1 resin · mo^-1). Maximum N availability as indicated by net N mineralization rates occurred in August (0.54 g N · m^-2 · mo^-1). Plants took up 96.1%, and soil microorganisms took up 3.9% of the ¹⁵N recovered from 12-d field incubations of ¹⁵NH₄⁺ in June; the corresponding percentages were 92.6% and 7.4% in August 1991. Thus, plants acquired N early in the season when they were actively growing, and the highest net microbial N immobilization occurred later in the season, after plant senescence. The potential for microbial competition for N may have been limited by: (1) constraints on microbial growth from the seasonal alpine freeze-thaw cycles, and (2) influences of roots on N cycling by soil microorganisms. The alternation between plant N uptake early in the season and microbial N uptake late in the season may enhance N retention in this N-limited ecosystem.